Since a magnesium alloy has low specific gravity and is light in weight, it can be widely used in a package of a mobile phone or a portable acoustic instrument, a car component, a machine part, a structural material and the like. Especially, in order to maximize the effect of light in weight, it is to be employed in a part of motor system or an operating system and more particularly, in a part of an engine system or driving system like a piston.
However, these parts and members require heat resistance characteristics around 200° C. in addition to the strength and toughness at room temperature. According to a conventional magnesium alloy, Mg—Al—Zn—Mn group alloy such as AZ91D alloy or Mg—Al—Mn group alloy such as AM60B alloy defined in JIS standard, for example, since its strength is lowered at a temperature above 120° C., it cannot be used in the above part.
In order to answer the needs for reduction in weight, an alloy in which the heat resistance characteristic of magnesium alloy is improved has been aggressively developed. For example, in “Materials Science Forum Vols. 419-422 (2003) pp. 425 to 432” in “Magnesium Alloys 2003” at Magnesium International Conference (Osaka International Conference Hall on Jan. 26 to 30, 2003), Mr. Y. Guangyin and the like announced that they developed a Mg—Al—Zn—Si—Sb-RE group alloy by a casting method and the alloy had tensile strength of 178 MPa and a breaking extension of 14% at 150° C. However, according to this alloy, since the average crystal grain diameter of magnesium that constitutes a matrix is 70 μm which is relatively large, its tensile strength is 235 MPa and breaking extension is 9% at room temperature, so that it cannot be applied to the above part.
Japanese Unexamined Patent Publication No. 2002-129272 discloses a Mg—Al—Zn—Ca-RE-Mn group magnesium alloy for die-casting that is superior in creep characteristics at high temperature around 150° C. Since the magnesium alloy disclosed in this document is manufactured by the casting method similar to the case by Mr. Guangyin and the like, the following problems are pointed out.
(1) The crystal grain of magnesium is as large as 60 to 150 μm.
(2) The compound such as Al11RE3, Al2Ca, and Mg17Al12 deposited and dispersed in the matrix grows to be coarse and becomes an acicular compound having a length of 20 to 40 μm or more.
(3) The acicular compound exist in a magnesium crystal grain boundary and when it is excessively formed, it exists like a network along the boundary.
As a result, there arises a problem such that it is inferior in strength and toughness at room temperature. Furthermore, when each element is added excessively in order to improve the tensile characteristics at high temperature, a problem such as fluidity or hot cracking is generated at the time of casting, so that the content of an additive element is limited and further improvement in heat resistance characteristics is not expected. For example, a magnesium alloy provided by die-casting disclosed in the Japanese Unexamined Patent Publication No. 2002-129272 is defined in its appropriate content within a range containing, by weight, 1 to 3% RE, 1 to 3% Ca, and 0.5 to 8% Al.
According to a high-strength magnesium alloy and a heat treatment method of a magnesium alloy cast disclosed in Japanese Unexamined Patent Publication No. 8-41576, it is described that a cast alloy containing, by weight, 1 to 4% Al, 1 to 8% RE, 0.3 to 1.3% Ca, 0.1 to 2% Mn and the balance Mg has superior creep characteristics. Furthermore, when a heat treatment such as solution treatment or ageing treatment is performed to the Mg alloy according to need, the characteristics are improved by enhancement of solid solution of Al or Ca and enhancement of deposition of Mg—Ca group compound.
However, since the magnesium alloy disclosed in the Japanese Unexamined Patent Publication No. 8-41576 is manufactured by the casting method, the Mg crystal grain is inevitably grown and becomes coarse during its solidification. As a result, since its tensile strength becomes 200 to 280 MPa at room temperature, it cannot be applied to a car equipment or a machine part or a structural material.
The inventor of the present invention found that the following conditions were required to implement both high strength and high toughness (extension) of the magnesium alloy within a temperature range from room temperature up to around 200° C.
(1) The crystal grain diameter of a magnesium alloy that constitutes a matrix is to be miniaturized.
(2) A compound that is superior in heat resistance is to be uniformly deposited and dispersed not as an acicular grain but as a fine grain.
(3) The above compound grain is to be dispersed in a magnesium crystal grain as much as possible.
(4) In order to deposit and disperse the fine compound superior in heat resistance as much as possible, it is effective to use a solid-phase (non-dissolved) manufacturing method not using a conventional casting or die-casting method but using a plastic forming method using powder or chips as a starting raw material.